280 VAN HISE—-METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
and the filling of the spaces produced by the reaction. A common illus- 
tration of this is the vesicular dolomite; however, in the deep-seated 
zone of rock flowage the sagging occurs and energy under (2) is liber- 
ated. 
I conclude from the foregoing that, in so far as energy is concerned, 
there are four cases: The chemical reaction may (1) release energy, and 
result in the development of heat ; (2) may consume energy, and result 
in absorption of heat. The change of volume may be (8) by compres- 
sion, and result in the development of heat, or (4) by expansion, and 
result in the absorption of heat. (1) and (8) will be called plus, and 
when they are combined the heat developed is equal to their sum; (2) - 
and (4) will be called minus, and when they are combined the heat 
absorbed is equal to their sum. When (1) and (4) or (2) and (8) are 
combined, heat may be developed or absorbed, depending upon the rel- 
ative values of the energy of the chemical reaction and that of the 
change of volume. 
As a case in which the reactions as to temperature and pressure are 
each in opposite senses in the upper and lower physico-chemical zones 
may be mentioned hydration and dehydration. The first process occurs 
in the upper zone, and represents an association which takes place with 
the great development of heat, while the second process occurs in the 
lower zone, especially in connection with mass dynamic action (see 
pages 306-310), and represents a dissociation which takes place with 
important absorption of heat. The first process results in very consid- 
erable expansion of volume and absorption of heat; the second process 
results in equivalent contraction of volume and development of heat. 
Therefore in the upper zone the first part of van’t Hoff’s law of chem- 
ical reactions dominates, and in the lower zone the law of pressure 
controls. 
The first part of this statement is sufficiently evident; the second pos- 
sibly needs some explanation. ‘To drive orf the combined water of rocks 
at ordinary pressures usually requires a temperature above 110°C. This 
temperature under mass static conditions would be found at a depth of 
about 3,300 meters. Itis certain that at depths less than this dehydration 
occurs (see page 309); hence I conclude that the increase of temperature 
does not produce the reaction. The volume of the hydrated solid is less 
than that of the residual solid plus the separated water; therefore, if the 
water could not escape, pressure would tend to preserve the combination. 
However, at any depth there is effective pressure tending to squeeze out 
both the free water between the particles .and the combined water, as 
one may squeeze the water from a sponge. During the process the com- 
bined water gradually joins the escaping free water. This effective 
